This invention relates generally microelectromechanical systems (MEMS). More particularly, forming landing pads on MEMS structures.
Background Art
Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers (known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate or rotate into the path of the light from the fiber. Mirrors that rotate into the light path generally rotate about a substantially horizontal axis, i.e., they xe2x80x9cflip upxe2x80x9d from a horizontal position into a vertical position. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these.
When the mirror is in the horizontal position, it rests against a substrate that forms a base. Often, the mirror is subject to electromechanical forces, sometimes referred to as xe2x80x9cstictionxe2x80x9d that cause the mirror to stick to the substrate and prevent the mirror from rotating. The same stiction forces can also prevent the mirror from being properly released from the substrate during manufacture. To overcome stiction problems, landing pads (also called dimples or bumps have been used extensively in MEMS devices to minimize or otherwise control the contact area between the device and the underlying substrate. In the prior art, such landing pads are formed prior to deposition of a device layer either by etching pits in an underlying sacrificial layer or by depositing pads of another material prior to the deposition of the layer forming the device.
Recently, silicon on insulator (SOI) techniques have been developed for fabricating MEMS devices. In SOI, an oxide layer is grown or deposited on a silicon wafer. A second silicon wafer is then bonded to the oxide layer, e.g. by plasma bonding. After bonding, the second silicon wafer is cleaved such that a thin layer of silicon is left attached to the oxide layer to form an SOI substrate. However, when that thin silicon layer is a MEMS device layer it is generally not possible to process the underside of the device layer prior to bonding the device layer to the oxide layer. Any processing of the device layer must therefore be done after it is attached to the underlying substrate. However since the underside of the device layer is firmly attached to the oxide layer it is not normally possible to deposit material on or etch material from the underside of the device layer. Currently, no technology exists for forming pads on the underside of the device layer of a MEMS device fabricated using SOI.
There is a need, therefore, for an SOI MEMS device having landing pads on an underside of the device layer and a method of fabricating same.
Accordingly, it is a primary object of the present invention to provide a controlled landing pad between a device layer and a substrate. It is a further object of the invention to control the material and shape of such a landing surface. It is an additional object of the invention to provide a method for fabricating such landing pads that is consistent with SOI fabrication techniques.
These objects and advantages are attained by a method for fabricating a landing pad structure on an underside of a device. The method comprises providing a substrate having a sacrificial layer disposed between a base layer and a device layer. One or more vias are etched through the device layer. The sacrificial layer is then partially etched to form one or more depressions in the sacrificial layer at locations corresponding to locations of the vias in the device layer. The vias and depressions are then filled with a landing pad material to form a structure having one or more landing pads protruding from an underside of the device layer. The sacrificial layer is subsequently removed to release the structure. Particular embodiments of both methods may be applied to fabricating microelectromechanical systems (MEMS) especially MEMS mirrors. In an alternative embodiment of the invention, the device layer is bonded to the sacrificial layer after forming the depressions in the sacrificial layer.
The various embodiments of the present invention produce an inventive device having a device layer with at least one landing pad on an underside of the device layer attached to the device layer by a plug passing through an opening in the device layer. The device may be attached to the device layer by one or more compliant flexures, which allow the device to rotate in and out of a plane defined by the device layer.
The various embodiments are well suited to use with silicon on insulator substrates since the patterning of a sacrificial oxide layer may be performed either before or after bonding the device layer to the rest of the substrate.